Rotating asynchronous converter and a generator device

ABSTRACT

A rotating asynchronous converter for connection of AC network with equal or different frequencies employs a first stator connected to a first AC network with a first frequency and a second stator connected to a second AC network with a second frequency, and a rotor which rotates in response to the first and second frequencies. The converter has at least one winding formed of a cable, including a conductor and a magnetically permeable, electric field confining insulating covering surrounding the conductor.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a rotating asynchronous converter.

The present invention also relates to a generator device.

BACKGROUND OF THE INVENTION

In a number of situations exchange of power must be performed between ACnetworks with different or at least not synchronous frequencies. Themost frequent cases are the following:

-   -   1. Connection of not synchronous three phase networks with equal        rating frequencies, e.g. between eastern and western Europe.    -   2. Connection of three phase networks with different        frequencies, most usually 50 Hz/60 Hz (e.g. Japan, Latin        America).    -   3. Connection of a three phase network and a low frequency,        one/two phase network for railway supply, in Europe 50 Hz/16.2/3        Hz, in USA 60 Hz/25 Hz.    -   4. The use of rotating asynchronous converters as a series        compensation in long distance AC transmission.

Today, the connection is performed with the aid of power electronics andDC intermediate link. In the above mentioned cases 2 and 3 theconnection can further be performed with the aid of matrix converters.In case of synchronous, but different frequencies in the above mentionedcases 2 and 3 the connection can further be performed with the aid ofrotating converters comprising mechanically connected synchronousmachines.

In the article, “Investigation and use of asynchronized machines inpower systems”, Electric Technology USSR, No. 4, pp. 90-99, 1985, by N.I. Blotskii, there is disclosed an asynchronized machine used forinterconnection of power systems, or their parts, which have differentrated frequencies, or the same rated frequencies, but differing in thedegree of accuracy with which it must be maintained. The structure ofthe asynchronized machine is disclosed in FIG. 1. The asynchronizedmachine includes an electric machine 1 which is a machine with aconventional three-phase stator and either a non-salient-polesymmetrical rotor or a salient-pole or non-salient-pole electricallyasymmetrical rotor, the phase leads being connected to slip rings; anexciter 2 which is a cycloconverter or reversing controlled rectifier,the cycloconverter supply 3 or 4, a regulator 5 forming the control lawrequired for the rotor ring voltages and the main machine rotor angleand speed 6, voltage 7 and current 9 sensors of the stator and rotor.

In the article, “Performance Characteristics of a Wide Range Inductiontype Frequency Converter”, IEEMA Journal, Vol. 125, No. 9, pp. 21-34,Sep. 1995, by G. A. Ghoneem, there is disclosed an induction-typefrequency converter as a variable frequency source for speed controldrives of induction motors. In FIG. 2 there is disclosed a schematicdiagram of the induction-type frequency converter. The induction-typefrequency converter consists of two mechanically and electricallycoupled wound rotor induction machines A, B. The stator windings of oneof them (A) are connected to 3-phase supply at line frequency (Vi, Fi),while the stator windings of the other machine (B) represent thevariable frequency output (Vo, Fo). The rotor windings 10, 12 of the twomachines are connected together with special arrangement. The converteris driven by a variable speed primemover 14, a DC motor can be used.

Static converters have drawbacks such as relatively low efficiency (ca95%) owing to the losses in the semiconductors, harmonics which have tobe compensated with the aid of filters. The use of DC intermediate linksleads to the use of special converter transformers with very complexdesign. The fillers are leading to a great need of space for the totalassembly. Conventional rotating converters are not designed for highvoltages, so a transformer is needed at each side for the connection tothe AC network. The efficiency then becomes comparable to or even lowerthan the efficiency of a static converter.

SUMMARY OF THE INVENTION

The object of the invention is to solve the above mentioned problems andto provide a rotating asynchronous converter for connection of ACnetworks with equal or different frequencies. This object is achieved byproviding a rotating asynchronous converter.

Accordingly, the converter comprises a first stator connected to a firstAC network with a first frequency f₁, and a second stator connected to asecond AC network with a second frequency f₂. The converter alsocomprises a rotor means which rotates in dependence of the first andsecond frequencies f₁, f₂. At least one of the stators each comprise atleast one winding, wherein each winding comprises at least onecurrent-carrying conductor, and each winding comprises an insulationsystem, which comprises on the one hand at least two semiconductinglayers, wherein each layer constitutes substantially an equipotentialsurface, and on the other hand between them is arranged a solidinsulation.

According to another embodiment of the converter, it comprises a firststator connected to a first AC network with a first frequency f₁, and asecond stator connected to a second AC network with a second frequencyf₂. The converter also comprises a rotor means which rotates independence of said fist and second frequencies f₁, f₂. The stators eachcomprise at least one winding, wherein each winding comprises a cablecomprising at least one current-carrying conductor, each conductorcomprises a number of strands, around said conductor is arranged aninner semiconducting layer, around said inner semiconducting layer isarranged an insulating layer of solid insulation, and around saidinsulating layer is arranged an outer semi-conductor layer.

According to another embodiment of the converter, it comprises a firststator connected to a first AC network with a first frequency f₁, and asecond stator connected to a second AC network with a second frequencyf₂. The converter also comprises a rotor means which rotates independence of said first and second frequencies f₁, f₂. The stators eachcomprises at least one winding, wherein each winding comprises at leastone correct-carrying conductor. Each winding also comprises aninsulation system, which in respect of its thermal and electricalproperties permits a voltage level in said rotating asynchronousconverter exceeding 36 kV.

A very important advantage of the present invention is that it ispossible to achieve a connection of two not synchronous networks withoutthe further use of transformers or any other equipment. Anotheradvantage is the high efficiency, which is expected to be 99%.

By designing the insulation system, which suitably is solid, so that itin thermal and electrical view is dimensioned for voltages exceeding 36kV, the system can be connected to high voltage power networks withoutthe use of intermediate step-down-transformers, whereby is achieved theabove referenced advantages. Such a system is preferably, but notnecessarily, designed in such a way that it comprises the features ofthe rotating asynchronous converter.

Another object of the invention is to solve the above mentioned problemsand to provide a generator device with variable rotational speed. Thisobject is achieved by providing a generator device.

Accordingly, the generator device comprises a stator connected to an ACnetwork with a frequency f₂, a first cylindrical rotor connected to aturbine, which rotates with a frequency f₁. The generator device alsocomprises a rotor means which rotates in dependence of the frequenciesf₁, f₂. The stator and the first cylindrical rotor each comprises atleast one winding, wherein each winding comprises at least onecurrent-carrying conductor, and each winding comprises an insulationsystem, which comprises on the one hand at least two semiconductinglayers, wherein each layer constitutes substantially an equipotentialsurface, and on the other hand between them is arranged a solidinsulation.

According to another embodiment of the generator device, it comprises astator connected to an AC network with a frequency f₂, and a firstcylindrical rotor connected to a turbine, which rotates with a frequencyf₁. The generator device also comprises a rotor means which rotates independence of the frequencies f₁, f₂. The stator and the firstcylindrical rotor each comprises at least one winding, wherein eachwinding comprises a cable comprising at least one current-carryingconductor, each conductor comprises a number of strands, around saidconductor is arranged an inner semiconducting layer, around said innersemiconducting layer is arranged an insulating layer of solidinsulation, and around said insulating layer is arranged an outersemiconducting layer.

The above mentioned and other preferable embodiments of the presentinvention are specified in the dependent claims.

In a certain aspect of the present invention it relates to the use ofthe invented asynchronous converter in specific applications such asthose specified in claims 38-41, in which applications the advantages ofthe invented device are particularly prominent.

Embodiments of the invention will now be described with a reference tothe accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of an asynchronized machine used forinterconnection of power system according to the state of the art;

FIG. 2 shows a schematic diagram of an induction-type frequencyconverter as a variable frequency source according to the state of theart;

FIG. 3 shows the parts included in the current modified standard cable;

FIG. 4 shows a first embodiment of a rotating asynchronous converteraccording to the present invention;

FIG. 5 shows a second embodiment of the rotating asynchronous converteraccording to the present invention;

FIG. 6 shows a first embodiment of a generator device according to thepresent invention; and

FIG. 7 shows a second embodiment of the generator device according tothe present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

A preferred embodiment of the improved cable is shown in FIG. 3. Thecable 20 is described in the figure as comprising a current-carryingconductor 22 which comprises both transposed non-insulated 22A andinsulated 22B strands. There is an extruded inner semiconducting casing24 which, in turn, is surrounded by an extruded insulation layer 26.This layer is surrounded by an external semiconducting layer 28. Thecable used as a winding in the preferred embodiment has no metal shieldand no external sheath.

Preferably, at least two of these layers, and most preferably all ofthem, has equal thermal expansion coefficients. Hereby is achieved thecrucial advantage that in case of thermal motion in the winding, oneavoids defects, cracks or the like.

FIG. 4 shows a first embodiment of a rotating asynchronous converter 30according to the present invention. The rotating asynchronous converter30 is used for connection of AC networks with equal or differentfrequencies. The converter 30 comprises a first stator 32 connected to afirst AC network (not disclosed) with a first frequency f₁, and a secondstator 34 connected to a second AC network (not disclosed) with a secondfrequency f₂. In the disclosed embodiment the stators 32, 34 are threephase stators 32, 34 comprising three windings each, wherein eachwinding comprises at least one current-carrying conductor, and eachwinding comprises an insulation system, which comprises on the one handat least two semiconducting layers, wherein each layer constitutessubstantially an equipotential surface, and on the other hand betweenthem is arranged a solid insulation. The windings can also be formed ofa cable of the type disclosed in FIG. 3. The converter 30 also comprisesa rotor means 36 which rotates in dependence of the first and secondfrequencies f₁, f₂. In the disclosed embodiment the rotor means 36comprises two electrically and mechanically connected three phase rotors36 ₁, 36 ₂, which are concentrically arranged in respect of said stators32, 34. The converter 30 also comprises an auxiliary device 38 connectedto said rotors 36 ₁, 36 ₂ for starting up of the rotors 36 ₁, 36 ₂ to asuitable rotation speed before connection of said converter 30 to saidAC networks. Each rotor 36 ₁, 36 ₂ comprises a low voltage winding (notdisclosed). When the first stator 32 is connected to a three phase ACnetwork with the frequency f₁ and the second stator 34 is connected to athree phase AC network with the frequency f₂, the rotors 36 ₁, 36 ₂ willrotate with the frequency (f₁−f₂)/2 and the stator current has thefrequency (f₁+f₂)/2. The efficiency with such a converter will be veryhigh (˜99%) for small frequency differences due to the fact that allpower is transmitted as in a transformer. Assuming f₁<f₂, a proportion$\frac{f_{1} - f_{2}}{f_{2}}$of the power is transmitted mechanically and the remainder$\frac{f_{1}}{f_{2}}$of the power is transmitted by transformer action. Mechanical power isonly consumed to maintain the rotation.

In FIG. 5 there is disclosed a second embodiment of the rotatingasynchronous converter 40 according to the present invention. Therotating asynchronous converter 40 is also used for connection of ACnetworks with equal or different frequencies. The converter 40 comprisesa first stator 42 connected to a first AC network (not disclosed) with afirst frequency f₁, and a second stator 44 connected to a second ACnetwork (not disclosed) with a second frequency f₂. In the disclosedembodiment the stators 42, 44 are three phase stators 42, 44 comprisingthree windings each, wherein each winding can be of the type describedin connection to FIG. 4. The converter 40 also comprises a rotor means46 which rotates in dependence of the first and second frequencies f₁,f₂. In the disclosed embodiment the rotor means 46 comprises only onerotor 46 concentrically arranged in respect of said stators 42, 44. Saidrotor 46 also comprises a first loop of wire 48 and a second loop ofwire 50, wherein said loops of wire 48, 50 are connected to each otherand are arranged opposite each other on said rotor 46. The loops of wire48, 50 are also separated by two sectors 52 ₁, 52 ₂, wherein each sector52 ₁, 52 ₂ has an angular width of α. The converter 40 also comprises anauxiliary device (not disclosed) connected to said rotor 46 for startingup of the rotor 46 to a suitable rotational speed before connection ofsaid converter 40 to said AC networks. To compensate for the frequencydifference Δf, the rotor 46 only needs to rotate with the frequency${f_{R} = {\frac{\pi - \alpha}{\pi}\quad \cdot \quad\frac{\Delta\quad f}{4}}},$wherein Δf=|f₁−f₂|. For α=π/4 this means${f_{R} = \frac{3\quad\Delta\quad f}{16}},$a very low rotational frequency. The main advantages with thisembodiment are the low rotational frequency and the use of only onerotor.

In FIG. 6 there is disclosed a first embodiment of a generator device 60with variable rotational speed according to the present invention. Thegenerator device 60 comprises a stator 62 connected to an AC network(not disclosed) with a frequency f₂ and a first cylindrical rotor 64connected to a turbine 66, which rotates with a frequency f₁. Thegenerator device 60 comprises also a rotor means 68 which rotates independence of the frequencies f₁, f₂. The stator 62 and said firstcylindrical rotor 64 each comprises at least one winding (notdisclosed). Each winding comprises at least one current-carryingconductor, and each winding comprises an insulation system, whichcomprises on the one hand at least two semiconducting layers, whereineach layer constitutes substantially an equipotential surface, and onthe other hand between them is arranged a solid insulation. Each windingcan in another embodiment also comprise a cable of the type disclosed inFIG. 3. The rotor means 68 comprises two electrically and mechanicallyconnected rotors 68 ₁, 68 ₂, which rotors 68 ₁, 68 ₂ are hollow andarranged concentrically around said stator 62 and said cylindrical rotor64. The stator 62 in the disclosed embodiment has a cylindrical shape.The rotors 68 ₁, 68 ₂ each comprises a low voltage winding (notdisclosed) and they are rotating with the frequency (f₁−f₂)/2 when saidgenerator device is in operation. The frequency of the rotor currentwill be (f₁+f₂)/2 when the generator device 60 is in operation. Thisgenerator device 60 is now disconnected from the power frequency and canbe operated with the frequency as an optimizeable parameter. Thisgenerator device 60 will also give a better efficiency and powermatching than a conventional generator.

In FIG. 7 there is disclosed a second embodiment of the generator device70 according to the present invention. The generator device 70 comprisesa stator 72 connected to an AC network (not disclosed) with a frequencyf₂ and a first cylindrical rotor 74 connected to a turbine 76, whichrotates with a frequency f₁. The generator device 70 also comprises arotor means 78 which rotates in dependence of the frequencies f₁, f₂.The stator 72 and said first cylindrical rotor 74 each comprises atleast one winding (not disclosed). The winding can be of the types whichwere mentioned in the description in connection to FIG. 6. The rotormeans 78 comprises a first rotor 78 ₁ and a second rotor 78 ₂, whichrotors 78 ₁, 78 ₂ are electrically and mechanically connected to eachother. The first rotor 78 ₁ is hollow and arranged concentrically aroundsaid first cylindrical rotor 74 and said second rotor 78 ₂ iscylindrical and surrounded by the stator 72. The first and second rotors78 ₁, 78 ₂ of said rotor means 78 each comprises a low voltage windingand said rotors 78 ₁, 78 ₂ are rotating with the frequency (f₁−f₂)/2when said generator device 70 is in operation. The stator 72 is hollowand arranged around said second rotor 78 ₂. This generator device 70works in the same way and has the same advantages as the generatordevice 60 disclosed in FIG. 6.

The disclosed embodiments only show connection of three phase networks,but the invention is also applicable for connection of a three phasenetwork, wherein one stator has a one/two phase application. Theinvention can also be used for connection of a three phase network and aone/two phase network, wherein one stator having a three phaseapplication is connected via a Scott-connection or another symmetricalconnection to a one/two phase network. The invention is also applicableto more than two stators and rotor parts to connect more than two ACnetworks. The only condition is that only two not synchronous networksare connected.

The invention is not limited to the embodiments described in theforegoing. It will be obvious that many different modifications arepossible within the scope of the following claims.

1. A rotating asynchronous high voltage converter for connection of ACnetworks with equal or different frequencies, wherein the convertercomprises a first stator connected to a first AC network with a firstfrequency f₁, and a second stator connected to a second AC network witha second frequency f₂, wherein the converter comprises a rotor whichrotates in dependence of the first and second frequencies f₁, f₂, andwherein at least one of said stators includes at least one windingforming at least one uninterrupted turn, said winding including acurrent-carrying conductor a plurality of insulated conductive elementsand at least one uninsulated conductive element, and a magneticallypermeable, electric field confining insulating covering surrounding theconductor, including an inner layer having semiconducting propertiesbeing in electrical contact with the conductor, an insulating layersurrounding the inner layer being in intimate contact therewith and anouter layer having semiconducting properties surrounding the insulatinglayer and being in intimate contact therewith, wherein each layer formsa substantially equipotential surface.
 2. The rotating asynchronousconverter according to claim 1, wherein at least one of saidsemiconducting layers has substantially equal thermal expansioncoefficient as said solid insulation.
 3. The rotating asynchronousconverter according to claim 2, wherein the potential of the inner oneof said layers is substantially equal to the potential of the conductor.4. The rotating asynchronous converter according to claim 1, wherein theouter layer is arranged to form substantially an equipotential surfacesurrounding said conductor.
 5. The rotating asynchronous converteraccording to claim 4, wherein said outer layer is connected to aspecific potential.
 6. The rotating asynchronous converter according toclaim 5, wherein said specific potential is ground potential.
 7. Therotating asynchronous converter according to claim 1, wherein said innerand outer layers have substantially equal thermal expansioncoefficients.
 8. The rotating asynchronous converter according to claim1, wherein each of said inner and outer layers is fixedly connected tothe adjacent layer of solid insulation along substantially the whole ofa connecting surface therebetween.
 9. The rotating asynchronousconverter according to claim 1, wherein the winding comprises a cablehaving a diameter comprised in the approximate interval 20-250 mm and aconductor area comprised in the approximate interval 80-3000 mm². 10.The rotating asynchronous converter according to claim 1, wherein saidrotor comprises two electrically and mechanically connected rotors,which are concentrically arranged in respect of said stators.
 11. Therotating asynchronous converter according to claim 10, wherein saidconverter further comprises an auxiliary device connected to said rotorsfor starting up the rotors to a suitable rotation speed beforeconnection of said converter.
 12. The rotating asynchronous converteraccording to claim 11, wherein each of said rotors comprises a lowvoltage winding, and wherein said rotors are rotatable with thefrequency (f₁−f₂)/2 and the stator has a current with a frequency(f₁+f₂)/2 when said converter is in operation.
 13. The rotatingasynchronous converter according to claim 1 wherein said rotor comprisesa single rotor concentrically arranged in respect of said stators. 14.The rotating asynchronous converter according to claim 13, wherein saidrotor comprises a first loop of cable and a second loop of cable,wherein said loops of cable are connected to each other and are arrangedopposite each other on said rotor and separated by two sectors, whereineach sector has an angular width of α.
 15. The rotating asynchronousconverter according to claim 14, wherein said converter furthercomprises an auxiliary device connected to said rotor for starting upthe rotor to a suitable rotational speed before connection of saidconverter, and said rotor is rotatable with the frequency${f_{R} = {\frac{\pi - \alpha}{\pi}\quad \cdot \quad\frac{\Delta\quad f}{4}}},$wherein Δf=|f₁−f₂|.
 16. The use of a rotating asynchronous converter inaccordance with claim 1 for connection of non-synchronous three phasenetworks with equal rating frequencies.
 17. The use of a rotatingasynchronous converter in accordance with claim 1 for connection ofthree phase networks with different frequencies.
 18. The use of arotating asynchronous converter in accordance with claim 1 as a seriescompensation in long distance AC transmission.
 19. The use of a rotatingasynchronous converter in accordance with claim 1 for reactive powercompensation.
 20. A rotating asynchronous converter for connection of ACnetworks with equal or different frequencies, wherein the convertercomprises a first stator for connection to a first AC network with afirst frequency f₁, and a second stator for connection to a second ACnetwork with a second frequency f₂, rotor means rotatable in dependenceof the first and second frequencies f₁, f₂, and each stator includes atleast one winding forming at least one uninterrupted turn, said windingcomprising at least one current-carrying conductor a plurality ofinsulated conductive elements and at least one uninsulated conductiveelement, and a magnetically permeable, electric field confininginsulation system surrounding the conductor, including an inner layerhaving semiconducting properties being in electrical contact with theconductor, an insulating layer surrounding the inner layer being inintimate contact therewith, and an outer layer having semiconductingproperties surrounding the insulating layer and being in intimatecontact therewith, wherein each layer forms a substantiallyequipotential surface, which permits a voltage level in said rotatingasynchronous converter exceeding 36 kV.
 21. A generator device operablewith variable rotational speed, wherein the generator device comprises astator for connection to an AC network with a frequency f₂, a firstcylindrical rotor for connection to a turbine, rotatable at a frequencyf₁, wherein said generator device comprises rotor means being rotatablein dependence of the frequencies f₁, f₂, and said stator and said firstcylindrical rotor each includes at least one winding forming at leastone uninterrupted turn, said winding comprising at least onecurrent-carrying conductor a plurality of insulated conductive elementsand at least one uninsulated conductive element, and a magneticallypermeable, electric field confining insulation system, including aninner layer having semiconducting properties being in electrical contactwith the conductor, an insulating layer surrounding the inner layerbeing in intimate contact therewith and an outer layer havingsemiconducting properties surrounding the insulating layer and being inintimate contact therewith, wherein each layer forms a substantiallyequipotential surface surrounding the conductor.
 22. The generatordevice according to claim 21, wherein at least one of saidsemiconducting layers has substantially equal thermal expansioncoefficient as said solid insulation.
 23. The generator device accordingto claim 22, wherein the inner layer has a potential substantially equalto a potential of the conductor.
 24. The generator device according toclaim 22, wherein the outer one of said layers is arranged to formsubstantially an equipotential surface surrounding said conductor. 25.The generator device according to claim 24, wherein said outer layer isconnected to a specific potential.
 26. The generator device according toclaim 25, wherein said specific potential is ground potential.
 27. Thegenerator device according to claim 21, wherein at least two of saidlayers have substantially equal thermal expansion coefficients.
 28. Thegenerator device according to claim 21, wherein each of said two layersand said solid insulation is connected to adjacent layer or solidinsulation along substantially the whole connecting surface.
 29. Agenerator device with variable rotational speed comprising a stator forconnection to an AC network with a frequency f2, a first cylindricalrotor for connection to a turbine, being rotatable with a frequency f₁,wherein said generator device comprises rotor means including twoelectrically and mechanically connected hollow rotors arrangedconcentrically around said stator and said cylindrical rotor, beingrotatable in dependence of the frequencies f₁, f₂, and said stator andsaid first cylindrical rotor each comprises at least one winding,forming at least one uninterrupted turn, wherein each winding comprisesa cable including at least one current-carrying conductor, eachconductor comprises a number of conductive elements, an innersemiconducting layer surrounding the conductor and being in electricalcontact therewith, an insulating layer of solid insulation surroundingthe inner layer and being in intimate contact therewith, and anoutermost layer having semiconducting properties surrounding theinsulating layer and being in intimate contact therewith, wherein eachinner and outermost layer forms a substantially equipotential surfacesurrounding the conductor.
 30. The generator device according to claim29, wherein the cable has a diameter of about 20-250 mm and a conductorarea is about 80-3000 mm².
 31. The generator device according to claim29, wherein said rotor means comprises a plurality of insulatedconductive elements and at least one uninsulated conductive element. 32.The generator device according to claim 29, wherein each of said rotorscomprises a low voltage winding, and said rotor is rotatable at afrequency (f₁−f₂)/2 when said generator device is in operation.
 33. Thegenerator device according to claim 32, wherein said stator has acylindrical shape.
 34. The generator device according to claim 29,wherein said two rotors comprise a first rotor and a second rotor,wherein-said first rotor is arranged concentrically around said firstcylindrical rotor, and said second rotor is cylindrical.
 35. Thegenerator device according to claim 29, wherein said first and secondrotors of said rotor means each comprises a low voltage winding, andwherein said first and second rotors are rotatable at a frequency(f₁−f₂)/2 when said generator device is in operation.
 36. The generatordevice according to claim 35, wherein said stator is hollow and arrangedaround said second rotor.
 37. A rotating asynchronous converteremploying a high voltage electric machine comprising a stator, a rotorand a winding comprising a cable including at least one current-carryingconductor and a magnetically permeable, electric field confining coversurrounding the conductor and being in electrical contact therewith,said conductor including a plurality of insulated conductive strands andat least one uninsulated conductive strand in contact with the cover,said cable forming at least one uninterrupted turn in the correspondingwinding of said machine, and wherein said cover includes an innersemiconducting layer surrounding the conductor; and being in electricalcontact therewith, an outermost layer of solid insulation surroundingthe inner layer and being in intimate contact therewith, and anoutermost layer having semiconducting properties surrounding theinsulating layer and being in intimate contact therewith, wherein eachinner and outermost layer forms a substantially equipotential surfacesurrounding the conductor.
 38. The converter of claim 37, wherein theinner layer and outermost layer each have a conductivity sufficient toestablish an equipotential surface around the conductor.
 39. Theconverter of claim 37, wherein the inner layer and outermost layer havesemiconducting properties.
 40. The converter of claim 37, wherein theinner layer, insulating layer and outermost layer are substantially voidfree.
 41. The converter of claim 37, wherein the machine is operable at100% overload for two hours.
 42. The converter of claim 37, wherein thewinding is operable free of sensible end winding loss.
 43. The converterof claim 37, wherein the winding is operable free of partial dischargeand field control.
 44. The converter of claim 37, wherein the windingcomprises multiple uninterrupted turns.
 45. The converter of claim 37,wherein the cable is flexible.
 46. A rotating asynchronous converter forconnection of AC networks with equal or different frequencies, whereinthe converter comprises a first stator connected to a first AC networkwith a first frequency f₁, and a second stator connected to a second ACnetwork with a second frequency f₂, wherein the converter furthercomprises rotor means which rotates in dependence of said first andsecond frequencies f₁, f₂, said stators each comprise at least onewinding, wherein each winding comprise a cable including at least onecurrent-carrying conductor, and an electric field confining, solidinsulation covering surrounding the conductor, said conductor includingat least one of a plurality of insulated conductive elements and atleast one uninsulated conductive element in contact with the covering,said cable comprising an inner semiconducting layer surrounding theconductor, and being in electrical contact therewith an insulating layerof solid insulation surrounding the inner layer and being in intimatecontact therewith, and an outermost layer having semiconductingproperties surrounding the insulating layer and being in intimatecontact therewith, wherein each inner and outermost layer forms asubstantially equipotential surface surrounding the conductor.